The present invention relates to epitaxial reactors and, more particularly, to a gas collector for epitaxial reactors.
Continuing advances in the semiconductor industry have resulted in the development of highly complex thin-film deposition processes for fabricating semiconductor devices that are packaged for use in the manufacture of sophisticated electronic devices. The thin films of material that are deposited on the semiconductor wafers are often referred to as epitaxial layers. High speed electronic transistors, quantum-well diode lasers, light-emitting diodes, photodetectors, and optical modulators incorporate structures composed of numerous epitaxial layers ranging in thickness from several microns to as thin as a few tenths of a nanometer. These epitaxial layers are typically deposited, or grown, on a single-crystal substrate, i.e., the semiconductor wafer.
One method of forming epitaxial layers on a semiconductor wafer is known as chemical vapor deposition (CVD). In a typical manufacturing process of a wafer, for example, silicon or gallium arsenide in extremely pure crystalline form is overlaid sequentially with numerous layers of materials, which function as conductors, semiconductors, or insulators. Each subsequent layer is ordered and patterned such that the sequence of layers forms a complex array of electronic circuitry. The semiconductor wafer can then be subsequently cut along predetermined scribe lines into individual devices, commonly referred to as xe2x80x9cchips.xe2x80x9d These chips ultimately function as key components in electronic devices ranging from simple toys to complex supercomputers.
CVD processes normally take place within a reaction chamber. Initially, the semiconductor wafer is placed within a reaction chamber containing an inert atmosphere, and the temperature within the reaction chamber is elevated. Reaction gasses containing the compound or element to be deposited are then introduced to react with the surface of the semiconductor wafer, which results in deposition of the required film onto the semiconductor wafer. The reacted gasses are continually introduced and removed from the reaction chamber until a requisite film thickness has been achieved.
An example of an epitaxial reactor is described in U.S. Pat. No. 4,961,399, to Frijlink, which is incorporated herein by reference. This patent describes a reactor into which reaction gasses are introduced via a quartz funnel that is located at the center of the reactor. The reaction gasses then flow radially outward towards a quartz ring that bounds the reactor. Along the circumference of the quartz ring are equidistant slits, which collect the reacted gasses. Bounding the upper portion of the reaction chamber is a quartz disk. The quartz disk seals against O-rings, witch are positioned behind the quartz ring. Because quartz is a brittle and inflexible material, the quartz disk does not seal against the quartz ring. Instead, a gap is provided between the quartz disk and the quartz ring to prevent chipping of either.
This gap between the quartz disk and the quartz ring can cause problems within the reactor. For example, reaction gasses can escape through the gap and can form deposits outside the reaction chamber, and these deposits can interfere with the working of the reaction chamber and can also flake off and act as contaminants. Although a narrower gap can be provided, if a hard foreign body wider than the gap is introduced into the gap, such as during the opening of the reaction chamber, the foreign body could prevent the quartz disk from sealing properly over the reaction chamber or can cause chipping of either the quartz disk or the quartz ring.
An attempted solution to the above-described problems is disclosed in U.S. Pat. No. 4,976,217 to Frijlink, which is incorporated herein by reference. This patent describes a collecting crown or gas collector, which is both used to collect reaction gasses from the reaction chamber and also to provide a seal between the reaction chamber and a quartz disk or cover.
The gas collector and reaction chamber of the prior art is illustrated in FIGS. 1 and 2. The gas collector 1 is mounted on a supporting platform 4 by a horizontal plate 10 that rests upon the supporting platform 4. The supporting platform 4 is typically formed from quartz and is positioned within a cylindrical body 19 of the reactor that surrounds the reaction chamber and the gas collector 1. The cover 8 of the reaction chamber bounds the top of the reaction chamber and seals against the upper ridge 6 of the gas collector 1 and against toric joints 20 within the cylindrical body 19.
The gas collector 1 is further illustrated in FIG. 3. The gas collector 1 is formed from a folded plate of molybdenum having elastic properties. The molybdenum plate is folded along horizontal folding lines 13 and vertical folding lines 14 to form multiple flat plates 17, 5, 18, 9, 3, 10 that are connected to one another along the folding lines 13, 14. Also, two plates 2, 3 are touching without being fixed to each other. The combination of plates 17, 5, 18, 9, 3, 10 form a conduit 30 that encircles the reaction chamber. One of the plates 17 includes regularly spaced inlets holes 12 that collect the reaction gasses from the reaction chamber. Instead of the inlet hole 12, as shown below on the right-hand side of FIG. 3, the wall plate 17 can be provided with folded lower projections 15, which separate the movable lower edge 2 away from the fixed edge 3 to leave a slot between the edges 2, 3 through which the reaction gas can then pass.
The ""217 patent states that an essential element of the gas collector 1 is the vertical baffle plate, which is constituted by plates 17, 3 with the lower edge 2 of the upper plate 17 being pressed with a sliding motion against the upper edge of the lower plate 3. The horizontal plates 10 that are connected to the lower plates 3 serve to place the gas collector 1 on the edge of the platform 4. Furthermore, the top plate 5 is inclined and includes an upper ridge 6.
The disclosed gas collector 1 suffers several problems. A non-exhaustive list of these problems include contamination of the periphery 11 of the platform, the top plate 5, and the cylindrical body 19; uneven gas flow and gas density of the reaction gasses through the reaction chamber; and contamination within the reaction chamber. Many of these problems stem from the gas collector 1 being completely formed from a sheet of molybdenum, which is folded along folding lines 13, 14. Sheet metal structures are very difficult to manufacture to a high degree of dimensional precision. For example, the bending of the sheet metal along the folding lines 13, 14 is imprecise at best. Furthermore, the gas collector 1 is constructed using small screws and nuts, which do not lend themselves to maintaining a high degree of dimensional precision.
The gas collector 1 being formed by sheet metal, therefore, provides poor dimensional precision or tolerances for both the horizontal plate 10 extending over the platform 4; the positions of the inlets 12 in the upper plate 17; the connections of the upper plates 17 with one another, and the ridges 6 of the top plate 5. Another reason for the poor dimensional tolerances of the gas collector 1 results from thermal stressing of the sheet metal during the deposition process. As the thin molybdenum sheet metal of the gas collector 1 expands and contracts during each process cycle, the gas collector 1 eventually buckles and warps, thereby destroying the dimensional integrity of the gas collector 1.
The result of these poor dimensional tolerances is that the gas collector 1, although purporting to seal the reaction gasses within the reaction chamber except through the inlets holes 12, provides numerous locations for the reaction gasses to escape the reaction chamber. For example, the ridge 6 often fails to complete seal the gas collector 1 against the cover 8. As such, reaction gasses are free to flow past the ridge 6 and form deposits, for example, on the top plate 5, rear plate 18, and on the cylindrical body 19.
The deposits formed on the gas collector 1 and cylindrical body 19 require frequent cleaning of both the gas collector 1 and the cylindrical body 19. For example, in one application, the disclosed gas collector 1 was being cleaned after approximately every 20 process cycles. Furthermore, because the gas collector 1 is formed by molybdenum sheet metal, the deposits on the gas collector 1 are very difficult to remove without damaging the gas collector 1. This limits the number of cleanings of a particular gas collector 1, on average, to three times before the gas collector 1 is replaced.
A disadvantage of having deposits on the gas collector 1 is that the deposits can flake off and contaminate the inside of the reactor. These flakes can interfere with the deposition process on the semiconductor wafers and can cause the subsequent rejection of the wafers. With the disclosed gas collector 1 of the prior art, for example in one application, approximately 13.5% of the wafers are rejected for contamination caused by flakes.
The flakes are caused, for example, because the gas collector 1 is formed from molybdenum sheet metal. Molybdenum is a material onto which deposits cannot firmly adhere. As such, these deposits can easily flake off when stressed. Flexing of the molybdenum sheet metal creates the stresses within the deposits that cause the formation of the flakes or chips. The sheet metal flexes for several reasons, one of which is that the gas collector is formed from sheet metal, and sheet metal is notorious for flexing, which also relates to why constructs made from sheet metal have poor positional tolerances. A second reason is that the gas collector 1 is designed to be flexed. As stated above, the plates 3, 9, 18, 5 constitute a spring; and therefore, any deposits formed on the plates 3, 9, 18, 5 are subject to stress during the opening and closing of the cover 8. Still another reason for flexing is that molybdenum expands and contracts because of the heating and cooling of the gas collector 1 during a process cycle.
The reactor disclosed above in U.S. Pat. No. 4,961,399, with which the gas collector 1 of the prior art is used, is designed such that reaction gasses flow evenly from the center of the reaction chamber outward into the gas collector 1. A flow is considered even if the gas densities and velocities at a given radius away from the center of the reaction chamber are substantially equal. If the reaction gasses are not flowing evenly from the center of the reaction chamber, the deposition process varies depending upon the location of the wafers within the reaction chamber because the densities of the various constituents of the reaction gasses also vary. As such, the thickness and quality of the deposition can vary from one wafer to the next, even within the same batch process. For example, when depositing AlxGaAs using the gas collector 1 of the prior art, the percentage (x) of aluminum being deposited varies not only from one batch of wafers to the next, but also varies within wafers in single batch and also within a single wafer.
Obtaining an even flow of reaction gasses, however, is difficult with the gas collector 1 of the prior art. An even flow of reaction gasses results from the gas collector 1 providing an identical pressure differential between the reaction chamber and the conduit 18 inside the gas collector 1. As stated above, however, the gas collector 1 of the prior art is constructed with poor positional tolerances which provide gaps between the ridge 6 and the cover 8; gaps between adjacent front plates 17; and gaps between the horizontal plate 10 and the platform 4. Additionally, the holes used to form the bending lines 13, 14 also provide additional gaps in the gas collector 1. These gaps are not consistent along the circumference of the gas collector 10 and create different pressure differentials along the circumference, which therefore causes the reaction gasses to have different flow patterns depending upon the radial direction the reaction gasses flow.
Furthermore, the inlet holes 12 are positioned on a front plate 17 that is movable relative to the platform 4. This movement of the inlet holes 12 relative to the reaction chamber can change each time the cover 8 is raised and lowered and causes different flow rates that can vary during each batch process and/or from each gas collector 1. For example, the amount of pressure placed on the gas collector 1 when the cover 8 of the reactor is closed can vary, and this can cause the positions of the inlet holes 12 to vary. Also, for example, the positions of the inlet holes 12 can vary even if the pressure of the cover 8 remains the same because the flexibility of sheet metal forming the gas collector 1 varies over time. Furthermore, because the gas collector 1 of the prior art is made from sheet metal and is constructed used small screws, the flexibility or springiness of a particular gas collector 1 cannot be formed consistently, and therefore, the springiness varies from one gas collector 1 to the next. These positional variations of the inlet holes 12 cause the flow pattern of reaction gasses through the reaction chamber to change, and this change of the gas flow pattern affects the deposition process. Thus, the positioning of inlets 12 in a member movable relative to the reaction chamber causes an undesirable variance in the deposition process.
There is therefore a need for a gas collector that prevents the problems of the prior art, which include leakage of reaction gasses past the gas collector; flakes formed during the flexing of the gas collector; and uneven flow caused by the various gaps introduced into the gas collector.
This and other needs are met by embodiments of the present invention which provide a gas collector for collecting gasses from within a reaction chamber of a reactor. The gas collector includes a seal and a rigid body, in which is defined a conduit, at least one inlet, and an outlet. The seal cooperates with a removable lid of the reactor to prevent escape of the gasses from the reaction chamber. Also, the inlets direct the gasses from the reaction chamber into the conduit, and the outlet exhausts the gasses from the conduit into an exhaust pipe of the reactor. Furthermore the body can be formed from graphite.
By providing a rigid body, flexing of the body is reduced, which reduces the amount of flakes or contaminants generated within the gas collector. Furthermore, a rigid body also provides a more even flow through the gas collector by reducing variations in hole sizes and locations, which are also caused by flexing of the body. Additionally, because the rigid body can be formed to higher tolerances than a flexible body, leakage of reaction gasses past the gas collector can be reduced.
In one aspect of the invention, the seal is formed from molybdenum and is ring-shaped with a generally crescent-shaped cross-section and inner and outer edges. A top surface of the body can also include a pair of concentric slots into which the inner and outer edges are respectively positioned. The seal can also include symmetrically positioned slots around the outer edge of the seal.
In another aspect of the invention, the first and second members can be detachably connected to one another. Also, the first and second members can be stationary relative to one another during operation of the gas collector.